This invention relates to a method of and apparatus for ion implantation using a focused ion beam (FIB) for the fabrication of semiconductor devices.
This invention also relates to a liquid metal ion source (LMIS) for generating ions for microbeam ion implantation which is particularly suitable for semiconductor device fabrication.
In the conventional ion implantation technology used for fabrication of semiconductor devices, a broad ion beam ranging from several millimeters to several tens of centimeters in diameter is directed onto a substrate that is masked by a resist pattern.
Recently there has been progress in LMIS technology, in which a strong electric field extracts ions directly from a liquid metal surface. This technology offers the alternative of maskless microfabrication techniques such as milling, etching, resist exposure, ion implantation, and deposition to be performed in regions on the order of 0.1 micrometers in size without using a mask.
In applications to the fabrication of silicon-based large-scale integrated circuits, however, it is essential that the LMIS material include: (i) a dopant element (such as B, As, or P); (ii) a metallic element (such as Pt, Ni, or Pd) for electrode formation; and (iii) an element needed for observation of a scanning ion microscope (SIM) image of the region to be implanted, preferably an element (such as Si) that will not affect the device characteristics. Among the elements mentioned above, As and P have high vapor pressures and vaporize easily, while B and Pt have high melting points, yielding a small ion current density, so it is not possible to use these elements alone as the LMIS material. The necessary elements are therefore combined into a eutectic alloy in which the above faults are removed. LMIS alloys such as Au-Si, Au-Si-Be, Pd-Ni, Si-Be-B, Pt-As, Pd-As-B, Ni-B-Si, Pd-As, B-P, and Cu-P have been reported in the literature Journal of Vacuum Science and Technology B, Vol. 5, No. 2, 1987, p. 469. The materials used as an LMIS alloy are required to be easily wetted by but not to undergo chemical reactions with the emitter needles in a molten state.
Even when a eutectic alloy of elements such as Pd, Ni, B, As, Si and P is used as the LMIS material, however, it has not been possible to obtain a sufficient ion current because B and As are insufficiently fused in the alloy, causing As to be preferentially vaporized and B to be segregated. A further problem is that B, Ni and Pd undergo strong chemical reactions with emitter materials such as W, Ta, and Mo, so it is not possible to obtain an ion current that is stable for long periods of time.
Another problem is contamination of the LMIS by sputtered atoms or residual gas during ion source operation that has been found to reduce the ion current. Particularly, carbon adheres to the emitter surface during LMIS operation in an FIB apparatus and causes ion current to decrease by obstructing the flow of alloy on the emitter surface.